1. Field of the Invention
The present invention relates to a semiconductor laser device and, particularly, to a semiconductor laser device for use in a light pick-up device which reads information recorded on an optical recording medium.
2. Description of the Related Art
A light pick-up device which reads an optical signal on an optical memory device such as a CD-ROM or an MD (mini-disk) comprises a semiconductor laser device of a hologram laser system. The hologram laser system employs a semiconductor laser element, a hologram element and a signal light receiving element incorporated in a single package, and is adapted to project a light beam from the semiconductor laser element toward a disk (optical recording medium), diffract the light beam reflected from the disk by the hologram element, and guide the light beam to the light receiving element disposed apart from an optical axis.
The following devices are known to be related to the present invention.    (1) A semiconductor laser device comprising at least a semiconductor laser chip provided on a lead frame sealed with a resin material, the resin material containing particles of an insulative material having a high thermal conductivity (see, for example, Japanese Unexamined Patent Publication No. 11-25465 (1999)).    (2) A semiconductor laser device comprising a chip mount on which a semiconductor laser element is mounted, a frame surrounding the chip mount, and two projections provided on the frame in opposed relation with the chip mount held therebetween (see, for example, Japanese Unexamined Patent Publication No. 2001-111159).    (3) A semiconductor laser device comprising a chip mount on which a semiconductor laser element is mounted, a frame surrounding a surface of the chip mount on which the semiconductor laser element is mounted, two projections provided on the frame in opposed relation with the chip mount held therebetween, and an optical element provided on the frame between the two projections (see, for example, Japanese Unexamined Patent Publication No. 2000-196177).    (4) A package comprising a frame surrounding a semiconductor element mount, the frame having an outer peripheral surface including a surface formed by axially cutting away a part of a cylindrical surface (see, for example, Japanese Unexamined Patent Publication No. 2000-196176)    (5) A semiconductor laser device comprising a semiconductor laser chip provided on a chip mount, outer leads provided around the chip mount and having contacts connected to electrodes of the semiconductor laser chip, and a frame of an insulative material surrounding the chip mount and the contacts of the outer leads (see, for example, Japanese Unexamined Patent Publication No. 6-203403 (1994)).    (6) A semiconductor laser package for a hologram laser unit, the package comprising a semiconductor laser chip as a light source, a PIN photodiode for monitoring a laser beam emitted from the semiconductor laser chip, and a photodiode for reading signals on an optical disk, which are provided on a single planar stem and covered with a cap on the stem, and a glass plate formed with a diffraction grating for polarization and provided on an upper surface of a cap, wherein the stem has a generally elliptical plan shape such as defined by two opposed arcs and two opposed chords by removing two opposed segments from a circle, and the arcs of the stem of the generally elliptical shape serve as a reference member which determines a light emitting point of the semiconductor laser chip (see, for example, Japanese Unexamined Patent Publication No. 6-5990 (1994)).
These conventional semiconductor laser devices attempt to satisfy the following requirements:    (1) Reduction of the package size of the semiconductor laser device for realization of a pick-up device, a CD drive or a CD player having a reduced size and thickness; and    (2) Improvement of the positioning accuracy of the semiconductor laser device in the pick-up device for facilitating the adjustment of three beams by rotation.
However, none of the conventional semiconductor laser devices sufficiently satisfy these requirements.